A DC-to-DC converter, as known in the art, provides an output voltage that is a step-up, a step-down, or a polarity reversal of the input voltage source. Certain known DC-to-DC converters have parallel power units with inputs coupled to a common DC voltage source and outputs coupled to a load, such as a microprocessor. Multiple power-units can sometimes reduce cost by lowering the power and size rating of components. A further benefit is that multiple power units provide smaller per-power-unit peak current levels, combined with smaller passive components.
The prior art also includes switching techniques in parallel-power-unit DC-to-DC converters. By way of example, power units may be switched with pulse width modulation (PWM) or with pulse frequency modulation (PFM). Typically, in a parallel-unit buck converter, the energizing and de-energizing of the inductance in each power unit occurs out of phase with switches coupled to the input, inductor and ground. Additional performance benefits are provided when the switches of one power unit, coupling the inductors to the DC input voltage or to ground, are out of phase with respect to the switches in another power unit. Such a “multi-phase,” parallel power unit technique results in ripple current cancellation at a capacitor, to which all the inductors are coupled at their respective output terminals.
It is clear that smaller inductances are needed in DC-to-DC converters to support the response time required in load transients and without prohibitively costly output capacitance. More particularly, the capacitance requirements for systems with fast loads, and large inductors, may make it impossible to provide adequate capacitance configurations, in part due to the parasitic inductance generated by a large physical layout. But smaller inductors create other issues, such as the higher frequencies used in bounding the AC peak-to-peak current ripple within each power unit. Higher frequencies and smaller inductances enable shrinking of part size and weight. However, higher switching frequencies result in more heat dissipation and lower efficiency. In short, small inductance is good for transient response, but large inductance is good for AC current ripple reduction and efficiency.
The prior art has sought to reduce the current ripple in multiphase switching topologies by coupling inductors. For example, one system set forth in U.S. Pat. No. 5,204,809, incorporated herein by reference, couples two inductors in a dual-phase system driven by an H bridge to help reduce ripple current. In one article, Investigating Coupling Inductors in the Interleaving QSW VRM, IEEE APEC (Wong, Feb. 2000), slight benefit is shown in ripple reduction by coupling two windings using presently available magnetic core shapes. However, the benefit from this method is limited in that it only offers slight reduction in ripple at some duty cycles for limited amounts of coupling.
One known DC-to-DC converter offers improved ripple reduction that either reduces or eliminates the afore-mentioned difficulties. Such a DC-to-DC converter is described in commonly owned U.S. Pat. No. 6,362,986 issued to Schultz et al. (“the '986 patent”), incorporated herein by reference. The '986 patent can improve converter efficiency and reduce the cost of manufacturing DC-to-DC converters.
Specifically, the '986 patent shows one system that reduces the ripple of the inductor current in a two-phase coupled inductor within a DC-to-DC buck converter. The '986 patent also provides a multi-phase transformer model to illustrate the working principles of multi-phase coupled inductors. It is a continuing problem to address scalability and implementation issues of DC-to-DC converters.
As circuit components and, thus, printed circuit boards (PCB), become smaller due to technology advancements, smaller and more scalable DC-to-DC converters are needed to provide for a variety of voltage conversion needs.